Transplantation-associated arteriosclerosis (TAA) is the major cause of morbidity and death in recipients who survive more than one year after heart transplantation. The diffuse, concentric lesions of TAA represent a series of highly specific cellular and molecular responses that can be viewed, in aggregate, as an inflammatory disease process. Experimental, pathologic, and clinical observations support a critical role for the monocyte in TAA. Indeed, monocytes have been identified in every phase of neointimal development of the coronary arteries of the donor heart. However, despite the importance of the monocyte, our understanding of the mechanisms regulating its development remains poorly understood. Members of the Kruppel-like family of factors (KLFs) are known to play essential roles in erythrocyte and T lymphocyte development. Using a homology screening strategy we identified a member of this family termed KLF4 as being expressed in primary monocytes and in a monocytic restricted pattern among hematopoietic cell lines. In primary bone marrow cells, KLF4 is absent in common myeloid progenitors but highly expressed in monocytes but not granulocytes. Retroviral overexpression of KLF4 alone in the human promyelocytic cell line, HL-60, confers the morphologic, genetic (M-CSFR, CD11b, CD14), and functional (adhesion and phagocytosis) characteristics of a mature monocyte. Similarly, forced expression of KLF4 in primary common myeloid progenitors (CMPs) induced exclusive monocyte differentiation in colony forming assays, Conversely, KLF4 knockdown in HL-60 cells impaired monocyte differentiation. Our preliminary studies indicate that KLF4 induces monocyte differentiation in part by synergizing with Sp1, whereas it inhibits granulocyte differentiation through an antagonistic interaction with C/EBP-a. Finally, overexpression of KLF4 confers monocytic functions such as adhesion and phagocytosis. These observations serve as the basis for the central hypothesis that KLF4 modulates TAA by virtue of its ability to regulate monocyte development and function. The studies outlined in this proposal will (1) explore the mechanistic basis for KLF4's ability to regulate monocyte gene expression and development using the monocyte-specific CD14 promoter as a model system;and (2) as a consequence assess the role of KLF4 overexpression on experimental TAA in vivo using transgenic KLF4 mice.